Science doesn't really aim to answer the 'why'-questions, but rather the 'how'-questions. The scientific method boils down to falsifying hypothesis, and it's a lot easier with 'how does the tide work?' than 'why does the tide work (the way it does)?'.
Science can't say anything about 'Why does anything at all exist rather than not exist?', because there is no way to test any of the answers. So it's left to mythology, religion or philosophy to answer.